This invention generally relates to a process for thermally stress-relieving a selected portion of a metallic conduit, such as the U-bend section or a welded section of a heat exchanger tube formed from Inconel.RTM. 600 of the type used in nuclear steam generators.
Processes for stress-relieving metallic tubes are known in the prior art. These processes might be used, for example, to relieve the tensile stresses which may be induced across the wall of a metallic tube when the tube is either bent around a radius, radially expanded, or welded. Such stress-causing bends are incorporated into the heat exchanger tubes used in nuclear steam generators during their manufacture in order to give them their distinctive U-shape. Stress-causing expansions are routinely generated in the sections of these heat exchanger tubes that extend through the generator tubesheet, both during the manufacture and maintenance of the generator. Finally, stress-causing welds are placed around the interior walls of these tubes whenever reinforcing sleeves are welded therein.
Unfortunately, the tensile stresses that result from bending, expanding or welding the tube walls may lead to an undesirable phenomenon known as "stress corrosion cracking" if these stresses are not relieved. However, in order to fully understand the dangers associated with such stress corrosion cracking, and the utility of the invention in preventing such cracking, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are comprised of three principal parts, including a secondary side, a tubesheet, and a primary side which circulates water heated from a nuclear reactor. The secondary side of the generator includes a plurality of U-shaped heat exchanger tubes, as well as an inlet for admitting a flow of water. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in the tubesheet that hydraulically separates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends (see FIG. 1A). Hot, radioactive water flowing from the nuclear reactor is admitted into the section of the primary side containing all of the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets, up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water from the reactor transfers its heat through the walls of the U-shaped tubes to the nonradioactive feed water flowing through the secondary side of the generator, thereby converting feed water to nonradioactive steam that in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
The walls of the heat exchanger tubes of such nuclear steam generators can suffer from a number of different forms of corrosion degradation, one of the most common of which is intragranular stress corrosion cracking. Empirical studies have shown that the heat exchanger tubes may be more susceptible to stress corrosion cracking wherever they acquire significant amounts of residual tensile stresses, whether by bending, radial expansion, or welding. Where bending is concerned, the smaller radiused U-bends contain higher residual stresses and thus are more susceptible to stress corrosion cracking. These tubes are located near the center of the tubesheet (i.e., what are known as the "row 1" and "row 2" tubes). Tubes in row 1 have bend radii as small as approximately two inches. Applicants have recently found that a significant percentage of these centrally located heat exchanger tubes have exhibited stress corrosion cracking, primarily at the tangent-point where the semi-circular "elbow" of the U-shaped bend melds in with the straight-leg sections of the tube (see line "T" in FIG. 1B). Where tube expansions are concerned, such stress corrosion cracking has been displayed where the tubing has been radially expanded in order to minimize the annular clearance between the outer walls of the tube, and the holes bored through the tubesheet that receive the tubes. Here, it has been found that the cracking has manifested itself most frequently in what are known as the "transition zones" of the expansion, or the tapered sections of the tubes where the expanded portion melds in with the unexpanded portion of the tube (see no. 19 in FIG. 1B). Where welding is concerned, it has been found that such stress corrosion cracking may occur in the heat affected zone on either side of a circular weld joining a reinforcing sleeve to the inner wall of a heat exchanger tube (see No. 19.3 in FIG. 1B).
If such stress corrosion cracking is not prevented, the resulting cracks in the tube can cause the heat exchanger tubes to leak radioactive water from the primary side into the secondary side of the generator, thereby radioactively contaminating the steam produced by the steam generator.
In order to prevent such corrosion and tube cracking from occurring in the U-bend, expanded sections and welded sections of the heat exchanger tubes, various mechanical stress-relieving processes have been developed. One example of such a process is disclosed in U.S. Pat. No. 4,481,802 invented by Mr. Douglas G. Harmon et al. and assigned to the Westinghouse Electric Corporation. In this process, a shaft having a peening strip affixed thereto is inserted into a heat exchanger tube and rotated. The small peening balls attached to the rotating peening strip act as tiny hammers against the inner walls of the tube, and serve to relieve any residual tensile stresses therein. Processes for thermally stress relieving the stressed sections of such heat exchanger tubes are also known in the prior art. In such processes, the stressed section of the tubing is heated to a temperature sufficient to bring the tube walls into a plastic state, thereby allowing the microstructure of the walls to shift and to relieve any stresses contained therein.
Unfortunately, such prior art stress-relieving processes are not without limitations. While mechanical stress-relieving processes such as rotopeening have proven to be effective in relieving the stresses in the transition sections of the bottom portions of the heat exchanger tubes that have been expanded against the bores of a tubesheet, and might also be used where sleeves have been welded onto the interior walls of the tubes, such processes are difficult to apply to the U-bend sections of these tubes. Since the tubes are often about thirty feet in length, it is difficult (if not impossible) to effectively feed and drive a flexible peening shaft all the way up to and over the U-bend section of the heat exchanger tube. These problems are compounded when one attempts to bend a flexible rotopeening shaft around the smallest radiused U-bends that are the most needful of stress relief. The problems associated with mechanical stress relief led the applicants to consider thermally stress-relieving such U-bend sections. However, such thermal processes suffer two drawbacks. First, up until recently, there was no known heater capable of applying the necessary heat thirty feet up into the tube adjacent to the U-bend section in a practical manner. However, this problem has been solved by the recent invention of the flexible radiant heater described and claimed in U.S. Ser. No. 864,619 filed May 16, 1986, by John M. Driggers, Bruce Bevilacqua and Thomas Saska, and assigned to the Westinghouse Electric Corporation. The second drawback associated with such processes was the long amount of time it would take to apply enough heat to the U-bend section of the heat exchanger tube before the stresses within it are effectively relieved. It is known that the application of temperatures between 1,000.degree. and 1,100.degree. F. for about an hour are capable of relieving the tensile stresses in tubing formed from Inconel.RTM. 600. While the use of higher temperatures could significantly reduce the heating time, the prior art indicates that such temperatures might adversely affect the microstructure of the Inconel.RTM. 600 alloy used in such tubes, and thereby negate the benefits associated with stress relief. For example, it is known that the tensile stresses in a section of Inconel.RTM. 600 may be removed if the tube section is heated to 1500.degree. F. for a period of about 15 minutes. But under such conditions, some heats (or batches) of Inconel.RTM. 600 exhibited an enlarged grain growth as a result of such heating, which indicates a heightened susceptibility to corrosion as well as a reduction in mechanical properties. The exposure of Inconel.RTM. 600 to temperatures higher than 1500.degree. F. has been shown to remove certain carbide precipitates from the grain boundaries of the metal, which also indicates a heightened susceptibility to corrosion.
Accordingly, there is a need for a thermal stress relieving process that is capable of effectively relieving the tensile stresses in the remote, small radiused U-bend sections of the Inconel.RTM. heat exchanger tubes used in steam generators in a manner that is both rapid and effective. Such a method should be easy and inexpensive to implement, and capable of accurately, uniformly and reliably heat treating either the U-bend sections of these tubes or their transition zones or welded sections regardless of differences in their thermal loss properties or metallurgical properties. Finally, since there may be as many as eighty different heats of Inconel.RTM. 600 tubing in the forty miles of tubing typically used in a nuclear steam generator, the process should not be sensitive to the small but significant differences in metallurgical properties between different heats.